1. Field of the Invention
The invention relates to a circuit for converting AC voltage into DC voltage for at least one device, comprising
two external input terminals for supplying an AC voltage,
a rectifier module having two inputs connected via connection lines to the two external input terminals, and two output lines with external output terminals for coupling to the at least one device, and
at least one induction coil coupled in at least one of the connection lines between the external input terminals and the rectifier module.
2. Description of the Related Art
Many electric or electronic devices need a DC voltage for their operation, this DC voltage being made available or generated, respectively, from the AC voltage of the electric line voltage. A television set (TV) is an example of such a terminal device. The circuits for converting AC voltage into DC voltage installed in such devices need to satisfy certain requirements with regard to their conversion behavior, particularly, with regard to the remaining portion of the network harmonics. Such requirements are known, more particularly, from the subject matter of standards, for which, especially, CEI/IEC 61000-3-2, SECOND EDITION, 2000-08 may be mentioned, which is to be satisfied in Europe as from the year 2001 by devices that have a power consumption exceeding 75 W.
To guarantee that the quality standards are adhered to, there are various possibilities with various optimal power ranges. Active solutions are predominant for large powers of about 500 W and upwards. In the range of smaller powers, on the other hand, often 50 Hz coils are found. They are comparatively inexpensive, but also comparatively large and heavy. This size or this concentrated volume, respectively, leads to problems with the available space on printed circuit boards or to problems with shock resistance of the manufactured devices.
These problems particularly occur in applications which contain more than one switched-mode power supply. An example of this is formed by modern TV sets in which a second power supply is necessary for standby operation and, in normal operation, supplies power to the logic ICs.
With the above in mind, it is an object of the present invention to provide a circuit for converting AC voltage into DC voltage for at least one device, this circuit guaranteeing sufficient quality of the conversion with limited required space and expenditure, and being particularly suitable for applications having two switched-mode power supplies.
This object is achieved by a circuit described above, characterized in that the circuit comprises at least a second rectifier module having two inputs each connected to one of the external input terminals while bypassing the induction coil, and two output lines for connecting to the at least one device, the output lines including a switched-mode power supply arranged between the second rectifier module and the device to be connected.
The circuit for converting AC voltage into DC voltage for at least one device accordingly comprises the following component parts:
(a) Two external input terminals (EA) to which the poles of an external AC voltage source can be connected.
(b) A rectifier module of known structure having two inputs connected to one of the external input terminals via connecting lines, and two output lines for connecting to at least one device.
(c) At least one induction coil arranged in one of the connecting lines connecting the external input terminals to the rectifier module. The induction coil leveling the recharging current or reducing the harmonic part of the recharging current, respectively.
The circuit is characterized in that at least a second rectifier module is present having two inputs connected to a respective external input terminal without an induction coil being arranged in the associated connecting line. Furthermore, the rectifier module has two output lines for connecting to at least one device. A switched-mode power supply with a respective drive is then arranged, for this purpose, in the output lines, this switched-mode power supply having at least a substantially sinusoidal input current. This means that the input current has a small harmonic part. Such power supplies, which are also called AC/DC converter or PFC (Power Factor Correction circuit), are known from the literature (compare Siemens xe2x80x9cSchaltnetzteilexe2x80x9d W. Hirschmann, A.Hauenstein, 1990, ISBN 3-800-1550-2, chapters 6.1 to 6.3).
In the circuit according to the invention, the power supplied from the exterior as AC voltage is led over two separate inputs paths. One path leads via the first rectifier module. This path includes, in known manner, an induction coil for effecting a reduction of the harmonic part of the recharging current. In addition, according to the invention, a second input path is provided via the second rectifier module. The consequent relief of the first input path when the total power consumption remains the same, may be used for, accordingly, arranging smaller induction coils in this path, because the necessary inductance can be chosen smaller. In this way, it becomes possible to use induction coils of drastically reduced size which can also be used when there is limited space on printed circuit boards and to avoid problems with the shock resistance of the devices. Furthermore, the pattern of the power consumption from the AC voltage source, this pattern being caused by the circuit according to the invention, is modified in an advantageous manner, so that standard specifications can be satisfied in simple fashion at reduced cost.
Whereas, basically, different topologies can be used for the switched-mode power supply, this power supply is preferably arranged so that it can be operated with a sinusoidal current. For this purpose, preferably an up-converter (also called boost converter or forward gain controller) and/or a flyback converter are used, for contrary to many other topologies, the up-converter and the flyback converter can generate a purely sinusoidal input current when accordingly driven in a xe2x80x9cnormalxe2x80x9d fashion.
As an alternative, the switched-mode power supply (for example up-converter or flyback converter) can also be operated not to generate a purely sinusoidal input current, but to use the recharging pulse to keep the input current tapped from the external terminals sinusoidal as long as possible.
The circuit according to the invention may be complemented in various ways to obtain improved output signals. For example, the two output lines of the first rectifier module may be coupled each via a smoothing capacitor. This smoothing capacitor provides a suppression of higher frequencies in the output voltage.
The coupling via the output capacitor of the switched-mode power supply (for example, of the up-converter or flyback converter) takes place in the second output path.